Bismaleimides and their use in preparing polyimides

ABSTRACT

Bismaleimides of the formula I ##STR1## wherein R denotes a radical of the formula II ##STR2## and polyimides which can be prepared from these bismaleimides, if appropriate with the addition of an aromatic diamine. The polyimides are optionally reinforced with fibers.

DESCRIPTION

The invention relates to bismaleimides and optionally fiber-reinforcedpolyimides prepared therefrom.

Polyimides based on bismaleimides are already known from U.S. Pat. No.3,380,964, and they can be prepared by heating the bismaleimides attemperatures of 80° to 400° C., ethylene-bis-maleimide,diphenylmethane-bis-maleimide, diphenyl ether-bis-maleimide or diphenylsulfone-bis-maleimide, inter alia, being used.

The known polyimides are distinguished by a heat stability which is veryhigh for plastics. However, they have the disadvantage that they arevery brittle and can be further processed only to a limited degree. Forexample, only hard and brittle laminates can be produced from them.

The object of the invention was therefore to discover dismaleimideswhich can be polymerized to flexible and less brittle polyimides whichare optionally reinforced with fibers and if appropriate can be broughtinto the final shape by subsequent compression molding.

The object of the invention has been achieved with the aid of novelbismaleimides which contain aromatics as chain elements bonded via --S--and --SOhd 2-- bridges and from which polyimides with improvedproperties can be prepared.

The invention accordingly relates to bismaleimides of the formula I asfollows ##STR3## in which R denotes a radical of the formula II asfollows ##STR4##

The bismaleimides according to the invention are prepared by reactingthe diamine of the formula III as follows

    H.sub.2 N--R--NH.sub.2

with maleic acid or a maleic acid derivative. Preferably, the diamine ofthe formula III

    H.sub.2 N--R--NH.sub.2

is reacted with maleic anhydride. The reaction takes place particularlywell in polar solvents, such as, for example, dimethylformamide,dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone (NMP),tetramethylurea or hexamethylphosphoric acid triamide.

The diamine of the formula III, 1,4-bis-(4-aminophenylthio)diphenylsulfone (BDS) is obtained, for example, in accordance withER-A-0,245,815 by reaction of the Na salt ofbis-(4-mercaptophenyl)diphenyl sulfone with excess 4-chloronitrobenzeneto give 1,4-bis-(4-nitrophenylthio)diphenyl sulfone and subsequentreduction of the nitro groups.

The bismaleimides according to the invention can be used for thepreparation of polyimides with improved properties. The bismaleimidescan be polymerized either by themselves or together with an aromaticdiamine to give polyimides. Possible aromatic diamines are all thediamines known in polyamide and polyimide chemistry. Suitable diaminesare, for example, those based on aromatic, fused aromatic orheteroaromatic rings, which are optionally further substituted, forexample by alkyl or alkoxy groups or by halogen atoms. The diamine canbe built up from either one or more rings, the rings being bonded to oneanother directly or via bridge members. Examples of suitable bridgemembers are --O--, --CH₂ --, --C(CH₃)₂, --C(CF₃)₂ --, --S--, --SO--,--SO₂ --, --CO--, --CO--O--, --CO--NH--, --N═--, --NH--, --N(alkyl)--having 1 to 6 C atoms in the alkyl radical and --N(aryl)-- having 6 to20 C atoms in the aryl radical.

Examples of possible aromatic diamines are: 1,4-phenylenediamine,4,4'-diaminodiphenyl, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylsulfone, 4,4'-diaminodiphenyl ether, 4,4'-diaminobenzophenone,4,4'-methylenebis(o-chloroaniline), 1,2-bis(2-aminophenylthio) propane,diisopropyl methylenedianthranilate,4,4'-methylene-bis-2,6-diisopropylaniline, propyl3,5-diamino-4-chlorobenzoate,4,4'-diamino-5,5'-dimethylphenyl-2,2'-disulfonic acid,5(6)-amino-1l-(4-aminophenyl)-1,3,3-trimethylindane,5,5'-diamino-2,2'-bipyridyl sulfide, 2,4-diethyltoluidinediamine,4,4'-diaminobisbenzophenone, dioxyphenylenebisaminopyridine and2,5-diamino-1,3,4-thiadiazole.

The diamine of the formula III

    H.sub.2 N--R--NH.sub.2

1,4-bis(4-aminophenylthio)diphenyl sulfone, is preferred.

The invention furthermore relates to the polyimides prepared from thebismaleimides according to the invention. The polyimides consist ofrecurring monomeric units of bismaleimides of the formula IV ##STR5##the polymer bond being via the double bonds of the maleimide radicalswhere bismaleimides are used, and addition of the NH₂ groups of thediamine onto the double bond of the maleimide radical also taking placewhere bismaleimide and diamine are used. Polyimides which are built upfrom 0 to 1 mole of diamine per mole of bismaleimide are preferred, amolar ratio of diamine to bismaleimide of 0.1:1 to 0.6:1 beingparticularly preferred. The viscosity of the polyimides according to theinvention is in the range from 100 to 1,000 mPas, measured in 50%strength by weight freshly prepared solutions in NMP at 23° C.(Brookfield viscometer, spindle 3).

The polyimides according to the invention have a good temperaturestability up to about 400° C. and very good mechanical and electricalproperties, thus, above all, a very good impact strength. The mechanicalproperties show no noticeable decline up to temperatures of about 250°C.

On the basis of their solubility, especially in the abovementioned polarsolvents, and on the basis of their fusibility in the not completelyhardened state, they can be further processed very easily, for exampleto fibers, films, sheets or moldings. At the same time, the polyimidescan advantageously be reinforced with fibers for certain used forfurther improvement of the mechanical properties, it being possible touse short fibers, fiber mats, woven fabric or unidirectional fiberrovings or laid fiber fabrics for reinforcement. The content ofreinforcing fibers in the composite is usually about 20-70% by weight.The fiber mats can consist either of cut fibers or of continuousfilaments deposited as a non-woven material, it being possible for thefibers and filaments to be either in a tangled position or aligned. Thefiber mats are preferably compacted mechanically, for example byneedling, stitching or quilting. The weight per unit area of the fibermats is usually 250-1,200 g/m², and that of woven fabrics is preferably50-300 g/m².

Glass fibers, carbon fibers or aramid fibers or mixtures of thesefibers, for example, can be used according to the requirements of themechanical properties. Carbon fibers or aramid fibers are used above allwhere special requirements are particularly imposed on the strength,rigidity and low specific gravity of the components produced from thepolyimides.

In many cases, it proves to be particularly advantageous not to hardenthe polyimides completely, so that they can still be further processed,that is to say still remain deformable, fusible and thermocurable, andif appropriate can still be reshaped into a finished component in asubsequent hot press molding operation. This is of importance in thecase of prepregs which consist of, for example, only partly hardenedpolyimides reinforced with woven fiber fabrics or rovings, are stable onstorage and can also still be hot press molded to the desired finishedcomponent months later.

The polyimides according to the invention are prepared by heating thebismaleimide of the formula I ##STR6## if appropriate together with anaromatic diamine, at temperatures of about 100° to 300° C. More or lesscomplete hardening of the polyimide resin takes place according to thetemperature chosen and the duration of the heating. Heating ispreferably carried out in two stages, the mixture being heated totemperatures of about 100° to 180° C. in the first stage, depending onthe duration of the heating, and a polyimide which is not completelyhardened being in this way obtained. For example, the bismaleimide anddiamine can be heated at 140° C. for 10 minutes or at 150° C. for 5minutes or at 165° C. for 2 minutes, in which case the startingmaterials sinter and partly polymerize. The starting materials areadvantageously ground before and after the sintering. If appropriate,the incompletely polymerized polyimide is brought together with fiberreinforcement, and can then be hardened completely in a second state byfurther heating at temperatures of 120° to 300° C. under a pressure of 1to 20 bar.

Complete hardening can be achieved, for example, during heat treatmentat 200° C. for 60 minutes, or at 250° C. for 30 minutes under a pressureof 15 bar. To achieve the optimum mechanical properties, subsequentconditioning for several hours is necessary. For example, conditioningcan be carried out at 200° C. for 48 hours or at 25° C. for 24 hours,under normal pressure.

To prepare fiber-reinforced polyimide resins, for example, thereinforcing fibers can be brought together with the bismaleimide and ifappropriate the diamine and the mixture can then be heated to form thepolyimide resin. It is also possible for the fibers to be broughttogether with the already partly hardened polyimide powder.Fiber-reinforced laminates or prepregs are produced, for example, byapplying the finely ground powder, for example from a hopper, onto afiber reinforcement web, or by impregnating the reinforcement web with asolution and subsequently heating the webs, if appropriate underpressure. The fiber-reinforced laminates and prepregs are particularlyadvantageously produced continunously. In this production, thepulverulent, partly hardened polyimide is metered, for example, onto thebelt of a twin-belt press and at the same time one or more layers of thefiber reinforcement, in the form of a woven fabric, a roving, a liningor a mat, is also allowed to run into the press. In the twin-belt press,the woven fabrics, rovings, laid fabrics or mats are impregnated withthe molten polyimide at temperatures of 100° to 300° C. under pressuresof 1 to 20 bar, complete hardening also being possible, depending on theresidence time and temperature. In the case of solvent impregnation, thereinforcement web is passed, for example, through a trough containing anapproximately 40-60% strength by weight solution of the partly hardenedpolyimide in a polar solvent, for example in N-methylpyrrolidone. Theamount applied is adjusted via a doctor blade and squeeze rolls. Theimpregnated webs are dried, for example, in a vertical or horizontaldrying tunnel. The only partly hardened fiber-reinforced polyimides(prepregs) can be processed or reshaped to a completely hardenedfinished component, for example to printed circuit boards, electricalcoil cores, components of combustion engines or equipment components forair and space travel, under the action of pressure and temperature in alater processing step.

EXAMPLE 1 a) Preparation of the bismaleimide

23.5 g (0.05 mol) of 1,4-bis(4-aminophenylthio)diphenyl sulfone (BDS)were dissolved in 80 g of dimethylformamide (DMF) in a 250 ml flask,while flushing with nitrogen, and the solution was cooled to -20° C. 9.8g (0.1 mol) of maleic anhydride (MA), dissolved in 20 g of DMF, werethen slowly added dropwise, while stirring, so that the temperature ofthe reaction mixture did not exceed -15° C. The mixture was then stirredat -15° C. for a further 45 minutes, after which 1.0 g of anhydrous Naacetate and 15 g of acetic anhydride were added to the orange-coloredsolution, which slowly turned brown, at -15° C. and the solution washeated to 55° C. and further stirred at this temperature for 1 hour.After cooling, the solution was added dropwise to a mixture of ice andwater, after which a brown precipitate separated out and was filteredoff with suction, washed with water and dried at 80° C. in vacuo.

28.4 g (90% of theory) of 1,4-bis(4-maleimidophenylthio)diphenyl sulfonehaving a melting point of 140° C. were obtained.

b) preparation of the polyimide

1 mol of the bismaleimide obtained according to Example 1a was groundwith 0.4 mol of BDS and the mixture was then sintered at 150° C. for 10minutes. The fusible polyimide obtained in this way was ground to apowder. The melting point was 135°-185° C. The viscosity of a 50%strength freshly prepared solution in NMP at 23° C. was 300 mPas(Brookfield viscometer).

c) production of a Laminate

40% by weight, based on the finished composite, of the polyimide powderobtained in Example 1b was applied to a 20×20 cm glass woven fabric (No.92626, Interglas) with a weight per unit area of 296 g/m². The systemwas then heated at 200° C. for 3 minutes under atmospheric pressure,pressed at 200° C. for 1 hour under a pressure of 15 bar and finallyconditioned at 200° C. for 48 hours.

A flexible laminate was obtained; the properties are summarized in Table1.

EXAMPLE 2 a) Preparation of the polyimide

The bismaleimide obtained according to Example 1a was sintered at 200°C. for 10 minutes. The fusible polyimide obtained in this way was groundto a powder. The melting point of the yellow powder as 125°-190° C.

b) Production of a Laminate

The polyimide powder obtained according to Example 2a was applied in anamount of 40% by weight to a glass woven fabric analogously to Example1c and the system was pressed at 200°0 C. under 15 bar for 1 hour andconditioned for 48 hours. The properties of the resulting laminate aresummarized in Table 1.

EXAMPLE 3 a) Preparation of the polyimide

1 mol of the bismaleimide obtained according to Example 1a was groundwith 1 mol of BDS and the mixture was then sintered at 150° C. for 10minutes. The fusible polyimide obtained in this way was ground to apowder. The melting point was 160°-220° C.

b) Production of a prepreg

40% by weight of the polyimide powder obtained according to Example 3awas pressed with 60% by weight of a glass woven fabric (No. 92626,Interglas) at 150° C. under a pressure of 15 bar for 1 minute to give aprepreg which can be further processed as a thermoplastic.

c) Production of a laminate

The prepreg obtained according to Example 3b was pressed at 200° C.under a pressure of 15 bar for 1 hour and then conditioned at 200° C.under atmospheric pressure for 48 hours. The properties of the resultinglaminate are summarized in Table 1.

EXAMPLE 4 a) Preparation of the polyimide

1 mol of the bismaleimide obtained according to Example 1a was groundwith 0.4 mol of 4,4'-diaminodiphenyl ether and the mixture was thensintered at 150° C. for 5 minutes. The fusible polyimide obtained inthis way was then ground to a powder. The softening range was 160°-300°C.

b) Production of a laminate

40% by weight, based on the finished composite, of the polyimide powderobtained in Example 4a was applied as a 50% strength solution in NMP toa 20×20 cm glass woven fabric (No. 92626, Interglas). The system wasthen heated at 200° C. under atmospheric pressure for 3 minutes, pressedat 200° C. under a pressure of 15 bar for 1 hour and finally conditionedat 200° C. for 48 hours. A laminate with the physical values of Table 1is obtained.

A laminate with the same properties was obtained when the polyimideobtained according to Example 4a was applied as a powder to the glassfiber woven fabric.

                                      TABLE 1                                     __________________________________________________________________________                     Example                                                                       1    2    3    4                                             __________________________________________________________________________    Bismaleimide:diamine                                                                     (mol:mol)                                                                           1:0.4                                                                              1:0  1:1  1:0.4                                         Diamine          BDS.sup.1                                                                          BDS.sup.1                                                                          BDS.sup.1                                                                          DAE.sup.2                                     Tensile strength                                                                         N/mm.sup.2                                                                          520  450  215  290                                           DIN 53455  %     5.2  2.1  7.0  1.5                                           Elongation                                                                    DIN 53455  N/mm.sup.2                                                                          28000                                                                              32000                                                                              15000                                                                              23000                                         Flexural E modulus                                                            DIN 53457  %      40   40   40   40                                           Resin content                                                                 Bulk density                                                                             g/cm.sup.3                                                                           1.69                                                                               1.72                                                                               1.70                                                                               1.71                                         Impact strength                                                                          J/m   950  1000 1200 1100                                          Din 53453 according                                                                      N/mm.sup.2                                                                          340  400  250  380                                           to Izod                                                                       Compressive strength                                                          DIN 53454  N/mm.sup.2                                                                           16   14   17   18                                           Delamination                                                                  resistance                                                                    ASTM-D-2345                                                                              Ohm · cm                                                                   6.10.sup.14                                                                        5.5.10.sup.14                                                                      5.10.sup.14                                                                        6.10.sup.14                                   Specific volume                                                               resistivity                                                                   ASTM-D-257 1kHZ  4.6  4.7  4.8  4.6                                           Dielectric constants                                                          ASTM-D-150 1kHZ  1.1.10.sup.-2                                                                      1.0.10.sup.-2                                                                      1.3.10.sup.-2                                                                      1.2.10.sup.-2                                 Dielectric loss                                                               factor                                                                        tan delta at 1kHZ                                                             __________________________________________________________________________     .sup.1 1,4bis(aminophenylthio)diphenyl sulfone                                .sup.2 4,4diaminodiphenyl ether                                          

We claim:
 1. A bismaleimide of the formula I ##STR7## wherein R denotesa radical of the formula ##STR8##
 2. The use of a bismaleimide asclaimed in claim 1 for the preparation of polyimides consisting ofrecurring monomeric units of a bismaleimide of the formula IV ##STR9##0to 1 mol of an aromatic diamine per mol of bismaleimide additionallybeing added onto the maleimide groups.